Commonly-assigned U.S. Pat. No. 4,814,609 to Wraight et al., which issued Mar. 21, 1989 and is incorporated herein by reference, describes a logging-while-drilling (LWD) tool for performing radiation-based measurements of formation density and porosity while a borehole is being drilled. The LWD tool generally includes a tubular body adapted for tandem placement in the drill string. The tubular body is provided with an upwardly opening passage and an interior chamber for accommodating the insertion and removal of a carrier containing one or more energy radiating sources. The carrier is loaded and unloaded into the LWD tool at surface with shielding equipment and in a manner that is described in detail in commonly-assigned U.S. Pat. No. 4,845,359 to Wraight, which issued Jul. 4, 1989 and is hereby incorporated herein by reference.
The carrier is provided with a fishing head at its upper end that extends upwardly into the tubular body's upwardly opening passage. Such fishing head is provided so that in the event the LWD tool should become stuck in the borehole, the carrier can be retrieved with a fishing tool deployed from the surface via a cable, down the flow path of the drill string, and into the LWD tool's upwardly opening passage. Once the fishing tool has grasped the fishing head, the cable is pulled on from surface with sufficient force to cause a retaining pin to shear off and allow the carrier to be freed from the LWD tool and brought to surface. A conventional fishing grapple such as that available from Otis Engineering Company of Dallas, Tex. has been used for this purpose.
It has been found that under certain adverse drilling conditions, retrieving the radiation source carrier from the LWD tool while it is downhole by using a conventional fishing tool can be difficult. For example, in a highly deviated well, the force applied at the surface might only be partially transferred to the carrier because of significant contact and associated friction between the cable and the interior of the drill string, and this amount of force remaining at the carrier's fishing head may not be enough to shear the retaining pin. Also, in very deep wells, because of the weight of the cable itself and because only a limited amount of force can be applied to the cable to begin with before it might break at the surface, the amount of force actually applied to the carrier's fishing head again might not be enough to shear the retaining pin. In such instances, wireline jars must be employed to free the carrier from the LWD tool. However, use of such jars may damage the carrier or separate the fishing head from the carrier, making it then difficult or impossible to grasp the fishing head and bring the carrier to the surface.
Another problem with prior art methods and apparatus for fishing for a downhole tool is that there is insufficient information at the well surface available to the operator concerning the progress and status of the carrier extraction process. In addition, there is lack of timely confirmation of when or if the extraction process has been successful.
Another feature of the LWD tool described in U.S. Pat. No. 4,814,609 is that it may operate either in a recorder or "real-time" mode, or both. The recorder mode is accomplished with an on-board recorder for recording the LWD measurements downhole for later retrieval or "down-loading" when the tool is returned to the surface. The real time mode is accomplished with a mud-pulse telemetry system that transmits the measurement information to the surface via sonic pulses created in the drilling fluid. In some instances, sufficient but sparse data are telemetered to the surface in real-time mode because of the limited bandwidth of the mud transmission medium. The term "sparse" is used here to mean that not all measured data is typically transmitted to the surface. For example, high density data is not routinely transmitted to the surface via the mud flow path, but is recorded on the on-board recorder. Unfortunately, if the LWD tool should become permanently stuck in the borehole and must be abandoned, the data recorded downhole is also lost forever.
In light of the prior art problems described above, it is a primary object of this invention to provide a method and apparatus for removing downhole apparatus from an LWD tool while downhole with improved controllability and observability characteristics.
Another object of the invention is to provide a downhole fishing apparatus that is capable of applying its own extracting force directly to the downhole apparatus to be retrieved, such as a carrier for radiation sources in a logging-while-drilling tool, for the purpose of releasably extracting it from securement to the LWD tool.
A further object of the invention is to provide a downhole fishing apparatus with a bi-directional communication link to a downhole LWD tool via a wireline cable for the purpose of controlling the operation of the LWD tool from surface and for retrieving recorded information.
Yet another object of the invention is to provide a downhole fishing apparatus with a bi-directional communication link to a downhole LWD tool for the purpose of monitoring the progress and status of the downhole extraction process being conducted by the fishing apparatus.